Rules of Fandom: What Are They?

I mean, there's the classic Rule 34 of course: "If it exists, there is porn of it."

Or Rule 63, which defines the entire trope of genderswap, basically states that any character can and will exist as a member of the opposite sex. Technically, Urban Dictionary defines it as "For any given male character, there is a female version of that character." But it works for female characters being redone as male, too, though that is admittedly rarer.

Those rules came out of the pre-established Rules Of The Internet that emerged from the 4chan boards, as most early things on the 'net did. But much of the others have become flexible over time, and fandom has devised our own takes on the idea. So while things like Rule 34 (and its corollary #35) or Rule 63 are well known, others are more obscure.

Like, I am certain there was another rule about crossovers. Specifically about how all fandoms would eventually be crossed-over with Highlander. Which has, imho, so far proved ridiculously true. And yet, not even all my creative Googling can't find it. I think it was either Rule 33 or Rule 68 (as in 'Revelations 6.8') but I'm turning up bupkis. Sound familiar to anyone else? Anyone? Bueller?

What about any other Rules of Fandom you know that I don't? There's gotta be something!